The phenomenon of magnetostriction has been widely employed in linear distance and position measuring devices. A magnet located near or around a magnetostrictive wire marks the location to be measured. Such devices can operate with either mechanical or electrical excitation. When an acoustical/mechanical strain propagating along the wire reaches the area of influence of the magnet, an electrical signal is generated in the wire. Conversely, when an electrical signal propagating along the wire reaches the area of influence of the magnet, a torsional strain is generated in the wire.
Such linear position detectors using a magnet mounted in a float have been utilized as liquid level detectors to provide an indication of a level of liquid within a tank, such as an underground tank. The position of the magnet, and hence, the liquid level, is determined as a function of time required for a torsional disturbance to propagate from one end of the wire through the area of influence of the magnet, in the case of mechanical excitation, or from the position of the magnet to a sensing apparatus located at one end of the wire in the case of electrical excitation.
Other types of magnetostrictive position measuring devices utilize a reflective termination at the foot end of the magnetostrictive wire. Such devices measure the difference between the propagation times of a pulse from the magnet position to the foot of the wire and reflected back to the head of the device and a pulse traveling on the wire directly from the magnet to the head.
As shown in U.S. Pat. Nos. 4,839,590; 5,017,867; 5,050,430; and 5,253,521, all of which are assigned to the assignee of the present invention, such magnetostrictive devices include an elongated, small diameter tube, typically made of stainless steel, on which a movable magnet is mounted for providing an indication of a liquid level. A head and cap are mounted on one end of the tube, typically above the liquid level, and enclose electronic components, such as terminal connections and a signal conditioning circuit used to supply or output signals to and from the magnetostrictive wire in the tube.
Some liquid level detection applications require probe lengths of more than 20 feet. Since such probes are typically constructed of a rigid brass or stainless steel tube, the length of the tube creates significant problems with respect to storage, shipping and installation of the probes. The long, straight probes make it difficult to install the probe in confined areas lacking sufficient clearance above the tank for insertion of the probe through a port in the tank. Additional problems are encountered when installing such long length probes in large above-ground tanks. Such tanks require the installer to carry the probe up a ladder on the exterior of the tank and then to insert the long probe through an opening in the top of the tank.
It is known to construct liquid level detection apparatus or probes with a flexible housing in the form of a magnetostrictive wire mounted within a flexible, plastic outer tube. However, the use of a flexible outer plastic tube does not meet the requirement of non-permeability to fluids. The use of plastic outer housings has resulted in the ingress of toxic, corrosive, and/or explosive fluids into the interior of the housing which interfere with the timed propagation of signals along the magnetostrictive wire.
Thus, it would be desirable to provide a liquid level detection apparatus having a flexible, non-permeable probe housing which permits long length probes to be easily installed in confined areas around tanks and, also, to be easily handled during storage, shipment, and installation.